(length-limit-or-forced-length ,ly:dimension? "An automatically
generated lyric extender is suppressed if it would be shorter than
this length. A forced lyric extender is given this length if
possible.")

Sorry, but I strongly dislike collapsing two properties into one, even
if the values for the two properties are always the same. So please
use `collapse-length' (`suppress-threshold', etc.) and
`forced-length'.

I'll implement whatever the majority agrees upon.
Nevertheless: two parameters would obfuscate the fact that
the internal logic would always decide to use only one of those
lengths and to ignore the other.
For the case of two parameters I like best:
(forced-length ,ly:dimension? "A forced lyric extender
is given this length if possible.")
(collapse-length ,ly:dimension? "An automatically generated
lyric extender is suppressed if it would be shorter than
this length.")
Paul?
cu,
Knut